1. Field of the Invention
The present invention relates to a catalyst for CO shift reaction by which hydrogen (H2) is formed from carbon monoxide (CO) and water vapor (H2O), and more particularly, to a catalyst for CO shift reaction, which exhibits high activity under the conditions where the space velocity is high and the amount of water vapor is small, and can be used for fuel cells and purifying exhaust gases from an internal combustion engines.
2. Description of Related Art
The CO shift reaction has been applied to the synthesis of ammonia, removal of CO from city gas, adjustment of CO/H2 ratio in the synthesis of methanol and oxosynthesis, or the like. And recently, the CO shift reaction has been also used to remove CO in fuel reforming systems of internal reforming fuel cells. As shown in Reaction equation 1, the CO shift reaction is the reaction of forming H2 from CO and H2O, and is also referred to as water gas shift reaction. 
Examples of the catalyst for promoting the CO shift reaction include Cuxe2x80x94Zn catalysts which were reported by Girdler and Dupont Companies in the 1960s, and have been widely used in plants or the like. And in W. Hongli et al, China-Jpn.-U.S. Symp. Hetero. Catal. Relat. Energy Probl., B09C,213 (1982), there is reported that the catalyst obtained by reducing a catalyst in which Pt is supported by an anatase titania carrier at about 500xc2x0 C. exhibits higher CO shift reaction activity.
In addition, it has been known that catalysts in which xcex3-Al2O3 supports noble metal such as Pt, Rh, Pd or the like have CO shift reaction activity. Furthermore, it has been also reported that catalysts in which xcex3-Al2O3 supports Cu have higher CO shift reaction activity, as compared to the catalysts in which xcex3-Al2O3 supports noble metal such as Pt, Rh, Pd or the like.
In fuel reforming systems of the internal reforming fuel cells for motor vehicles or other moving bodies, or exhaust gas purifying systems adapted to reform CO in exhaust gases of motor vehicles to H2, and reduce NOx adsorbed on catalysts using the reformed H2, the dimensions of catalytic reactors therefore are limited, and consequently, the catalysts for the CO shift reaction need to exhibit high activity even under the reaction conditions where the space velocity is high.
These conventional Cuxe2x80x94Zn catalysts, however, have the problem that the activity thereof is low under the reaction conditions where the space velocity is high. Accordingly, under the reaction conditions where the space velocity is high, such as those in the fuel reforming systems of the internal reforming fuel cells, or the exhaust gas purifying systems, it becomes difficult to convert CO to H2 efficiently.
And the reaction expressed by the reaction equation 1 is an equilibrium reaction, and accordingly, as the reaction temperature rises, the reaction in the direction of the left-directed arrow mainly occurs to block the conversion of CO and H2O to H2. Accordingly, if the reaction temperature is increased to improve the activity of the Cuxe2x80x94Zn catalyst under the reaction conditions where the space velocity is high, the conversion of CO and H2O to H2 does not occur efficiently.
Furthermore, where the catalyst for the CO shift reaction is used in the fuel reforming systems of the internal reforming fuel cells, or the exhaust gas purifying systems of motor vehicles, the reactor may become a high temperature atmosphere temporarily due to the use conditions. In such cases, the problem also arises that the particle size of Cu as the active site of the Cuxe2x80x94Zn catalyst, or Cu in the catalyst in which xcex3-Al2O3 supports Cu, readily becomes larger, and consequently, the activity of the catalyst decreases. The efficient conversion of CO and H2O to H2 becomes further difficult.
Furthermore, where the catalysts for CO shift reaction are used in the fuel reforming systems of the internal reforming fuel cells, as the concentration of H2O increases, the reaction of forming H2 readily proceeds in the reaction of the reaction equation 1. Accordingly, Cuxe2x80x94Zn catalysts or the like have been generally used under the conditions where the H2O/CO ratio is 2 or more.
However, in order to carry out this reaction in limited environments such as motor vehicles, water tanks capable of storing a large amount of water and a large-sized evaporator or the like are needed, and consequently, the device becomes undesirably huge. In addition, in order to supply water vapor, a large amount of energy is needed to evaporate water, and consequently, the energy efficiency of the overall system decreases. Accordingly, it is desired to carry out the reaction with water vapor of which the amount is as small as possible. However, when the H2O/CO ratio decreases in the conventional catalyst for CO shift reaction, the activity thereof decreases, and consequently, the obtained H2 is less than an equilibrium value.
Under these circumstances, it has been contemplated to use noble metals which are estimated to exhibit high activity, and to be stable in an elevated temperature atmosphere, as compared to a base metal. As described above, the activity of catalysts in which xcex3-Al2O3 supports noble metals such as Pt, Rh, Pd or the like is, however, lower than that of the catalysts in which xcex3-Al2O3 supports Cu. And it has been known that in the catalysts in which anatase titania as a carrier supports Pt, a strong metal support interaction (SMSI) occurs between Pt and anatase titania. Accordingly, where these catalysts are exposed to reaction gases in a low temperature region of 200xc2x0 C. to 400xc2x0 C., Pt is covered with one part of a carrier material due to the strong metal support interaction (SMSI, and consequently, active points reduce to lower the activity thereof remarkably.
It is an object of the present invention to provide practical catalysts which exhibit especially high CO shift reaction activity in a low temperature region where the reaction of CO and H2O results in a favorable conversion to H2 in equilibrium.
The catalyst for CO shift reaction in accordance with the present invention is characterized in that the catalyst includes a carrier composed of titania as a main component, a noble metal supported on the carrier, and a sulfur-containing material adhering to the carrier.
It is preferable that the sulfur-containing material adheres such that the amount of pure sulfur ranges from 0.01 to 2.0 weight % of the carrier. And it is preferable that the noble metal is platinum.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification.